With the usual methods of establishing a connection, a connection request is routed to the called subscriber by means of a directory number of the called subscriber which is entered therein. The directory number predetermines the path through the communications network and is evaluated step by step along the way, i.e. it first addresses the local network area and the subscriber exchange and then the access of the called subscriber within the number range of the subscriber exchange. In this method, number portability is only possible within the number range of a subscriber exchange.
U.S. Pat. No. 5,375,167 indicates the possibility of enabling number portability within a larger number range.
With the method described therein, the number ranges of several subordinated exchanges are managed by a superordinate exchange. When needed in this case, blocks of numbers are requested by one of the subordinated exchanges and are assigned by the superordinate exchange. The call is subsequently routed by the superordinate exchange in accordance with the established assignments.
However, with all of these methods it is not possible to provide call number portability between the subscriber access networks of different network operators in a deregulated communications environment in which several competing subscriber access networks of different network operators are available in the same geographical area.